In the field of computer-controlled medical devices, it is common to provide electrically isolated signal inputs for receiving electrical input signals from a patient. For example, computer-based programming devices for non-invasive programming of implantable cardiac pacemakers, cardioverters, defibrillators, and the like, often receive patient signals, such as surface EKG signals, from which the programmer can obtain information about the operation of the implanted device. Such a computer-based programming device may be powered by standard household current (e.g. 110-120 VAC), or by power derived from some other source, such as a battery or a telephone line. In order to ensure that there is no opportunity for the patient to receive an electrical shock due to a malfunction or short-circuit in the programmer, patient inputs are usually isolated from line-powered portions of the programmer circuitry by means of an isolation device. Typically, the input signals on one side of the isolation link are modulated (as by frequency modulation (FM), phase modulation (PM), pulse width modulation (PWM) or the like). The signal thus modulated is then passed across the isolation link, such as a conventional optocoupler/optoisolator device, commercially available from a number of semiconductor device manufacturers. At the other side of the isolation link, the signal is demodulated to restore its original format, and then converted to a digital representation using an analog-to-digital converter (ADC). The isolation link thus establishes a boundary between two portions of circuitry which are electrically isolated from one another, preventing the direct electrical conduction of electrical signals between the two isolated portions of circuitry.
A principle drawback of the prior techniques of electrical isolation of input circuits is the complexity of circuitry required to perform the demodulation and digitization of the modulated signal after it has passed across the isolation link. In addition, if the input signal to be modulated has high-frequency components, both the modulation and digitization circuitry must operate at correspondingly high rates in order to ensure that the original input signal can be accurately reconstructed upon digital-to-analog conversion and demodulation.
It is accordingly a feature of the present invention that analog electrical input signals received by a medical device are coupled to an electrical circuit and used to modulate an oscillating carrier signal in a manner in which the source of the electrical input signals remains electrically isolated from the electrical circuit.
It is a further feature of the present invention that after the modulated signal is passed across an isolation boundary, no demodulation is performed prior to digitization.
It is yet another feature of the present invention that the sampling rate for the digitization step is determined by the frequency content of the original analog input signal, not the frequency content of the modulated carrier signal.
It is yet another feature of the present invention that certain pre-processing of the original analog input signal prior to modulation is performed to extract information about certain high-frequency components of the input signal, such that the frequency of the modulated carrier signal and the sampling rate of the digitization may be minimized.